KVM: arm64: Add generic check for system-supported vCPU features

To date KVM has relied on kvm_reset_vcpu() failing when the vCPU feature
flags are unsupported by the system. This is a bit messy since
kvm_reset_vcpu() is called at runtime outside of the KVM_ARM_VCPU_INIT
ioctl when it is expected to succeed. Further complicating the matter is
that kvm_reset_vcpu() must tolerate be idemptotent to the config_lock,
as it isn't consistently called with the lock held.

Prepare to move feature compatibility checks out of kvm_reset_vcpu() with
a 'generic' check that compares the user-provided flags with a computed
maximum feature set for the system.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230920195036.1169791-2-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>